We have developed a "hormonal milieu bioassay" that can be used to measure the relative concentration, bioavailability, and net bioactivity of estrogens, progestins and their antagonists (e.g., tamoxifen) in serum or other extracts (e.g., saliva). This bioassay provides a rapid, cost effective means to evaluate if the mechanism of action of a known or uncharacterized drug or foodstuff is mediated through estrogen or progesterone receptor pathways and therefore has the potential of impacting the growth behavior of normal and malignant (e.g., breast and uterine cancers) sex hormone sensitive tissues. The bioassay is designed to distinguish estrogen and progestin agonists from antagonists. It utilizes several estrogen/progestin sensitive human breast cancer cell lines which when exposed to estrogens, antiestrogens, and/or progestins respond in a predictable manner that is unique to the concentration, bioactivity and type of hormone(s) present. Serum can be tested directly for hormonal activity without extraction. The bioassay is based on the effects of hormones on binding and receptor activation, end product induction and cell proliferation.